The invention relates to an air-compressing reciprocating piston-equipped internal combustion engine comprising at least one piston having a piston head with a frontal face; per piston, a cylinder surrounding the piston head; a rotation symmnetrical cavity being disposed approximately centrally in the frontal face of the piston head and constituting a combustion chamber: a fuel injection unit comprising a fuel injection nozzle having at least four orifices and adapted for injecting fuel directly into the cavity; and an air-feeding unit adapted for generating a rotary motion of the air charge entering the cavity; wherein the diameter of the cavity amounts to at least 55% of the diameter of the aforesaid cylinder.
In the case of known internal combustion engines, the preparation of the air-fuel mixture in the cavity constituting the combustion chamber occurs preponderantly due to the rapid rotary motion of the air charge in the cavity. This leads to a disintegration of the injected fuel jet into relatively small-sized droplets which are intermingled with the air. In particular, when the injected fuel quantities are large, the movement of the air must be a very intense one if a sufficient preparation of the mixture is to be achieved in a sufficiently short time. For this reason, the diameter of the combustion chamber-constituting cavity is generally chosen to be relatively small, so that the air swirl generated in the cavity by the air-feeding unit can be maintained or even increased. This suffers, however, from the drawback that much of the energy needed for preparing the air-fuel mixture is required in the form of the kinetic energy of the rotating air, and, due to this generation of an air swirl, there results a reduction of the air charge filling the internal combustion engine. This has a disadvantageous influence on the optimal power yield of the engine and its freedom from smoke formation, in particular in the case of free-suction engines.
Moreover, the intense air-flow which prevails in part also during the combustion step, results in an intensified heat transfer at the piston and thereby in a higher thermic stress acting on this part of the engine. A further drawback resides in that the manufacture of the unit for generating the rotary motion of the air, such as, for example, a spiral inlet duct, a tangential duct or an inlet valve with deflector suffers from relatively large tolerances, whereby the swirl of the air charge can vary greatly from one cylinder to another, and/or from one engine to another, respectively. This may partly result in swirl values which are either too high or too low, this may lead to a deterioration of the smoke emission and also of the fuel consumption of the engine.
It is also known to build air-compressing reciprocating piston-equipped internal combustion engines without an air intake unit generating a rotary motion of the air charge. In this type of construction, the distribution of the fuel in the cavity constituting the combustion chamber takes place via a considerable number of orifices in the fuel injection nozzle; in some cases the nozzle has ten or more orifices. It is almost exclusively this specific design of the injection nozzle which serves to prepare the air-fuel mixture. Internal combustion engines of this last-mentioned type comprise combustion space-constituting cavities having a very large diameter and a relatively low depth, and must be equipped with fuel injection means which are capable of delivering fuel at very high pressures of about 1000 to 1300 bar, in order to achieve the preparation of the air-fuel mixture within a sufficiently short time, in particular when large amounts of fuel are to be injected.
Such internal combustion engines suffer from the drawback of an excessively large noise level, the emission of nitrogen oxides and mechanical stress exerted on the piston. Moreover, the high injection pressures of such fuel injection units which comprise injection lines between the pump and the nozzles cause considerably problems of stress resistance; that is why most of the engines of this type can be operated only with pumps and nozzles which have been combined in a single unit.
In another internal combustion engine which has been described in German Offenlegungsschrift No. 27 38 687, the diameter of the cavity serving as combustion chamber amounts more than 55% of the diameter of the cylinder. However, the cavity in this type of engine is of particularly great depth, and different impingement levels in the cavity are to be attained by greatly varying the directions of the various fuel jets emitted by the fuel injection nozzle, in order to have the fuel impinge on the walls of the combustion chamber as uniformly as possible. No provision is made for a specific swirl, and no specific data are given with regard to the fuel injection unit, especially, as the object is exclusively to attain the distribution of the fuel over different levels in the cavity. As a matter of fact, an increased swirl is to be permitted in this particular arrangement.
Finally, it is known from GDR Pat. No. 205,488 to reduce the rotary speed of the combustion air in the combustion chamber, such air being required to span, during the injection period pertaining to the rated power output, about 60 to 80% of the areal sector formed by the axes normal to the orifices of two vicinal fuel jets. However, such measure is impossible to control, because there exists at present no known method of determining the rotary motion of the air in a combustion chamber while the engine is running during operation. Moreover, this teaching is intended to be applied to stationary engines or to internal combustion engines having a greatly reduced range of operational r.p.m.'s, which also explains the disclosed narrowly limited speed range of the combustion air. In the case of vehicle engines which have to cover a broad range of speeds, this teaching would not lead to satisfactory results because, at higher operational speeds, there would unavoidably occur a spreading of the air over neighboring fuel jets, which would cause increased smoke emission and fuel consumption.